Anomalous pseudogap formation in a nonsuperconducting crystal of Nd_1.85Ce_0.15CuO_4+y: Implication of charge ordering

Temperature variation of optical conductivity and Raman spectra has been investigated comparatively for oxygenated antiferromagnetic and reduced superconducting crystals of Nd_1.85Ce_0.15CuO_4+y. While the spectra of the reduced crystal change little with temperature, in the spectra of oxygenated crystal a conspicuous pseudogap structure evolves around 0.3 eV and activated infrared and Raman Cu-O phonon modes grow in intensity, with decreasing temperature from 340 K. The origin is proposed to be charge ordering instability induced by a minute amount of interstitial apical oxygen, which seems to be also responsible for the absence of superconductivity in an oxygenated (or as-grown) crystal.